Technological advances have led to complementary metal-oxide-semiconductor (CMOS) sensor images being leveraged for use in digital cameras, camcorders, video systems, and the like. CMOS is low cost and versatile and, thus, has become the technology of choice for many image sensor arrays. Within CMOS itself, many types of devices intended for visible imaging applications are in use. Such devices can be tailored to large-format still cameras, standard video cameras, and compact “web cam” units, for example, all with varying degrees of size, cost, and performance.
CMOS sensor images can include an integrated circuit with an array of pixel sensors, each of which can comprise a photodetector. Further, a CMOS sensor imager can be incorporated into a System-on-Chip (SoC), which can integrate various components (e.g., analog, digital, and so forth) associated with imaging into a common integrated circuit. For example, the SoC can include a microprocessor, microcontroller, or digital signal processor (DSP) core, memory, analog interfaces (e.g., analog to digital converter, digital to analog converters), and so forth.
Visible imaging systems implemented using CMOS imaging sensors can reduce costs, power consumption, and noise while improving resolution. For example, cameras can use CMOS imaging System-on-Chip (iSoC) sensors that efficiently merge low-noise image detection and signal processing with multiple supporting blocks that can provide timing control, clock drivers, reference voltages, analog to digital conversion, digital to analog conversion, and key signal processing elements. High-performance video cameras can thereby be assembled using a single CMOS integrated circuit supported by few components including a lens and a battery, for example. Accordingly, by leveraging iSoC sensors, camera size can be decreased and battery life can be increased. Also, dual-use cameras have emerged that can employ iSoC sensors to alternately produce high-resolution still images or high definition (HD) video.
A CMOS imaging sensor can include an array of pixel cells, where each pixel cell in the array can include a photodetector (e.g., photogate, photoconductor, photodiode, and so on) that overlays a substrate for yielding a photo-generated charge. A readout circuit can be provided for each pixel cell and can include at least a source follower transistor. The pixel cell can also include a floating diffusion region connected to a gate of the source follower transistor. Accordingly, charge generated by the photodetector can be sent to the floating diffusion region. Further, the imaging sensor can include a transistor for transferring charge from the photodetector to the floating diffusion region and another transistor for resetting the floating diffusion region to a predetermined voltage level prior to charge transference. A floating diffusion region of a pixel cell is commonly reset by opening a circuit to a reset voltage source. Such opening of the circuit can be managed by digital control.
A typical CMOS sensor records images on a frame by frame basis; the amount of light integrated during a particular frame is linearly dependent on the duration of each frame. Additionally, the duration of each frame is inversely related to the sensor frame rate such that faster frame rates allow less light to be integrated into each pixel. Various light integration modes can be employed by a CMOS imaging sensor. For instance, in full frame integration mode, each pixel can be integrated or exposed to a light source at almost any time during the duration of a full frame time except when the pixel is being read and reset. This mode can allow for the maximum amount of light to be integrated in each pixel, which can provide high signal integration. Further, in sub-frame integration mode, each pixel can be integrated or exposed to a light source for a period of time, which is less than a full frame time while maintaining the same frame rate as for the full frame integration mode.